It is proposed to study the mechanisms of light and dark-adaptation in Limulus ventral photoreceptors; specifically to further test the hypothesis that an increase in the intracellular calcium concentration mediates light-adaptation. Such an increase might effect sensitivity by binding at a regulatory site on the inner surface of the plasma membrane. The proposed research would test for the existence of such sites by measuring changes in surface potentials. Such measurements would be attempted by three independent methods: 1. by a technique which involves monitoring the voltage-dependent potassium conductance; 2. by a technique in which the waveform of the early receptor potential is monitored; 3. by a technique involving artificial carriers. Tests will be performed to determine if observed changes in surface potentials are due to changes in the concentration of intracellular calcium ions. It might then be possible to follow the time course of calcium binding to membrane sites, and to determine if the statistical occupation of these sites correlates with the sensitivity to light.